The invention relates to a discharge monitoring circuit for a store which stores the peak values of a periodic signal with varying amplitude, consisting of a store capacitor which is charged to the peak value of the signal in one period via a charging circuit, and which stores this peak value in the following period until the signal has reached a defined percentage value of the stored peak value as determined by a comparator circuit, in order to then be discharged to the instantaneous value of the signal with the aid of a discharge circuit.
Peak value stores for determining the voltage peaks of a periodic signal provide an analog voltage signal for processing within a measuring, controlling or regulating circuit. It is, for instance, possible to determine by measuring techniques rotational speeds using the output voltage of a low-pass mean value generator connected downstream from the peak value store, if the periodic signal is of saw-toothed configuration. Thus, certain controls which are dependent on rotational speed may be attained with the aid of comparator circuits. In other cases, it may prove necessary for an analog periodic signal which is to be regulated to always reach a constant peak value within each period so as to enable an analog actual value signal to be generated with the aid of a peak value store for determination of the respective peak and its processing in a regulating circuit.
Peak value stores in accordance with the state of the art operate according to the charging and discharging principle of a store capacitor which is charged to the current peak value of the signal of each period by a current source via a first comparator circuit and a diode, and stores this value in the following period until it is discharged to the instantaneous value of the signal via a further comparator circuit and a discharge transistor within the following period. The store capacitor is then charged again via the current source and the first comparator circuit to the voltage peak at the end of the following period.
This peak value store does, however, have the disadvantage that it can only store the peaks of the periodic signal to be stored if the percentage ratio of the peak of the instantaneous signal in relation to the peak value of the immediately preceding signal is not less than a certain value which can be set at the further comparator circuit. For reasons of compromise, this value may be approximately 70%. If this value is chosen very small, in order to enable the peak value store to follow again the dynamics of the voltage peaks, an unsatisfactory envelope curve signal is obtained at the output of the low-pass forming the mean value. If conditions during operation are such that the peak values of a period in relation to the peak value of the immediately preceding period are not below this settable value for quite a long time, the peak value store is operating correctly. If, on the other hand, the ratio of the immediately successive peak values of a periodic signal is smaller than this settable value even once, the voltage at the store capacitor is no longer able to follow the dynamics of the peak values of the periodic signal. The store capacitor then keeps its voltage at a constant value.